The present invention relates to the monitoring of the instantaneous of momentary state of an electric circuit able to operate in a limited range of two states, namely a normal state corresponding to the passage in a given conductor of the substantially nominal intensity and a critical or threshold state corresponding to the passage of a so-called threshold intensity in the said conductor. It applies more particularly to the case where the threshold intensity is zero and corresponds to the disconnection of the conductor. In this particular case, the process of the invention behaves in a manner of a process for the detection and signalling or notifying of the breaking of an electric circuit supplied with alternating or direct current. A particularly interesting application of a process of this type is in the detection of the breaking of filaments of electric lamps on a motor vehicle. In the remainder of the text, reference will more particularly be made to the latter application, although this is not limitative and many other circuits can be monitored at the same time.
In automobile technology, different systems have already been used for detecting as rapidly as possible the failure of one of the electric lamps constituting the equipment of a vehicle. The presently known systems comprise measuring either the voltage variations at the terminals of a resistor inserted in series in the supply circuit of a lamp to be monitored, or the variation in the permeability of a magnetic material under the action of a magnetic field created by the electric current passing through one of the conductors of the aforementioned circuits.
The first system requires the insertion into the circuit of one or more resistors having a perfectly controlled value which is sufficiently low that it does not dissipate too much power and sufficiently high to still give a usable signal. Systems of this type generally have the serious disadvantage of being unable to withstand the short-circuits of the member being monitored, the inserted resistors then acting as a fuse. This is a serious disadvantage, because whenever there is an intensity overload in the monitored circuit, it is necessary to envisage changing not only the member responsible for the overload but also the monitoring resistors which, due to the necessary high precision thereof, cannot be standard low cost articles.
The system based on the variation of the magnetic permeability of a probe has the disadvantage of requiring an alternating current excitation signal which it is necessary to produce especially because such a current does not necessarily exist on all automobiles. In addition, such systems are generally made from a solid magnetic material having the shape of a torus with a well controlled magnetization curve and in which there are one or more coils. They are relatively cumbersome and costly, particularly in the case when numerous members have to be monitored, as occurs on an automobile where there can be easily a total of a dozen electric lamps.
The present invention relates to a process for monitoring the instantaneous or momentary state of an electric circuit which solves in a simple and inexpensive manner the aforementioned problems. To this end, the invention uses an apparatus having magnetoresistors placed in the vicinity of a conductor traversed by the current to be monitored.
Procedures using such means are already known and are described more particularly in: U.S. Pat. No. 2,946,955 of July 26th 1960 entitled "Measuring apparatus comprising a magnetic field-responsive resistor as a condition-responsive element" and the article by K. H. KNICKMEYER, published in the I.B.M. Technical Disclosure Bulletin, Vol. 18, No. 8, January 1976, p. 2565 entitled "Magnetoresistive detection apparatus".
The apparatus described in the U.S. Patent uses the Hall effect or the magnetoresistance of semiconductors of the III-V group well known for their high mobility of their carriers (70,000 cm.sup.2 /Vs) and therefore their good qualities as magnetosensitive detectors. However, if it is recalled that at 1 mm from the axis of a conductor traversed by a current of 1A, the field is 20 e, the highest signals which can be expected with the most sensitive transducer of the type described in the said patent (type SV 130/I) is 0.35 mV. This is a very weak signal and in addition it constitutes an upper limit, because the smallest dimensions of the transducer being 4 mm the latter cannot be placed 1 mm from the conductor. Moreover, the materials used are very temperature-sensitive and no solution is proposed for solving this problem. The interference fields are entirely taken into account by the apparatus and appear to be the equivalent of a current. These problems limit the field of application of such apparatus to high currents, such as for example, those of electrolytic cells.
The system described in the I.B.M. Bulletin uses at least two magnetoresistors for measuring a single current. In the case of monitoring 4 bulbs, e.g. in a vehicle, it is necessary to have 4 bridges and 4 amplifiers. Moreover, the structure described does not make it possible to obtain independence of the influence of interference fields. Thus, any fields superimposed on the field created by the current to be measured, particularly the earth field, cannot be distinguished from the field to be measured. Finally, the temperature compensation is brought about only as a result of numerous complications. To this end, it is necessary to use two magnetoresistors which vary in the same way as a function of the temperature, but in opposite directions as a function of the field applied. However, the magnetoresistance is only sensitive to the amplitude of the field and not to its direction. To obtain a resistance variation dependent on the direction of the field, it is necessary to apply a polarization field corresponding to a 45.degree. magnetization rotation. Thus, the transducer response is significantly linked with the stability of the polarization point. However, if it is desired to retain a significant sensitivity, the polarization field is necessarily of the same order of magnitude as the field to be measured and consequently the earth field in certain cases. Such a system can only be used in conjunction with precise and therefore expensive shielding systems.